If you’re looking for a pictorial overview of gene transcription–the process of making a RNA copy of a gene–or just like making up posters to put on your walls, you might like to download this free poster (1.94 Mb PDF) from Nature Reviews Genetics by Vikki Weake and Jerry Workman.
As you can see it’s very large, so I can’t show you more than a small portion in a scale that you can read.
Although intended for scientists, if you are familiar with the basics of transcription this might give some idea of what scientists now think goes on. (There will–of course–be more to add!)
Older explanations often just show RNA polymerase (polymerase II) feeding DNA through it, generating a messenger RNA.
How polymerase II reads a gene is a complex and fascinating process in it’s own right, but there’s more to the ‘cycle’ of getting ready to read a gene (transcription initiation), processing the gene and completing transcription than just polymerase II.
Our DNA is packaged, wrapped around nucleosome cores made of eight histone proteins.
In DNA that is not being used the nucleosomes are compacted, packing away unused portions of the genome (above).
For genes that are to be read, or transcribed, this compacted chromatin is loosened but the nucleosomes are not removed.
The process of compacting and decompacting chromatin–DNA bound to nucleosomes–using a scheme of chemical modifications to tails that radiate from the cores of the nucleosomes.
Each of the eight histone proteins making up a nucleosome has a compact portion that is part of the core of the nucleosome and a flexible tail that sticks out from the nucleosome. Modifications of the histone tails have become known as the histone code, with different modification associated with different states the gene is in.
Nucleosomes are not cleared off the gene to read, but removed as needed. As the polymerase processes the gene, building the RNA copy of the gene, nucleosomes ahead of the polymerase are removed and later re-assembled once the polymerase has past (last picture).
The poster is an excellent visual overview of the process of transcription as we currently understand it. There is much more in the poster than I’ve written here; I invite readers to dive in and check it out for themselves.
In the interests of clarity (and my reputation!): Two days after writing this I got an email from Nature Genetics advertising this as a free poster, etc. I did not write this lifting it from NG’s PR machine! (I had hoped that I might bring at least one aspect of the science this poster covers to a slightly wider audience through a description of my own. That transcription needs to deal with nucleosome-laden DNA brings a much richer story to transcription.)
Other article from Code for life: